Method and apparatus for the production of metal powder

ABSTRACT

Provide a method and apparatus for producing, in an economical manner, metal powder offering high purity and comprising uniform particle shape and size. Produce metal powder of titanium metal, etc., using an apparatus that comprises a power supply for high-voltage/current discharge, a feeder of metal electrode made of titanium metal, etc., a high-voltage discharge generator equipped with a metal electrode made of titanium, etc., and its counter electrode, a water tank, a water inlet, an outlet for produced metal dispersion solution containing titanium metal, etc., a discharge pump, and an adjunct device for separating/recovering metal powder of titanium metal, etc., from the metal dispersion solution containing titanium metal, etc.

This application is the U.S. National Phase under 35 U.S.C. §371 ofInternational Application PCT/JP02/11026, filed on Oct. 24, 2002, whichclaims priority of Japanese Patent Application No. 2001-330583, filed onOct. 29, 2001. The International Application was published under PCTArticle 21(2) in a language other than English.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for producing, in aneconomical manner, metal powder offering a high purity of elementalmetal and comprising powder particles of uniform shape and size.

The invention also relates to a production of aforementioned metalpowder from titanium, zirconium, germanium, tin, gold, platinum andsilver, but applies chiefly to titanium powder production.

BACKGROUND OF THE INVENTION

Elemental metal materials, especially those offering a high purity ofelemental metal, are processed into various shapes and sizes accordingto the required applications, such as powder-molded products, sheetmetals, bars, thin wires and foil materials.

In recent years, metal powder offering high purity is drawing theattention as an effective molding material for use in various moldingprocesses such as powder metallurgy and thermal spraying. Powdermetallurgy is an important technology used in wide-ranging fieldsincluding the production of mechanical parts. Accordingly, demand formetal powder as a powder metallurgy material is also growing.

The traditional methods for producing metal powder include the classicmethod of mechanically and directly crushing metal particles into powderand the method to blow molten metal using gas and shape the blowndroplets into powder form. However, these methods posed problems such asirregularities in particle shape and size, poor economy, and so on.

Electrolysis is one of the relatively new methods known for producingmetal powder. However, electrolysis has been reported to produce metalof a brittle spongy or powder structure if metal deposition isimplemented under a condition outside the optimal range where metal of asmooth, fine and uniform crystalline structure can be deposited. Evenwhen deposition is achieved under an optimal condition, the metal powderobtained by the electrolysis method as presently known does not satisfythe required levels of purity or uniformity of metal particle shape andsize. Other problems associated with this methods, such as poor economy,also remain unresolved.

Among other metals, titanium is a relatively new metal compared withiron or copper that has been known since the ancient times or aluminum.Being lightweight and offering excellent strength and corrosionresistance under high temperatures, titanium metal is used in a widerange of industrial applications.

The examples include jet engine material, structural members and otherparts used in aircraft and spaceship, materials for heat-exchangers usedin thermal or nuclear power generation, catalyst materials for use inpolymer chemistry, and articles of daily use such as eyeglass frame andgolf club head. Titanium is also used in various other fields includinghealth products, medical equipment and dental materials, and theapplications of titanium are expected to grow further. Titanium isalready competing with stainless steel and duralumin, and is likely tobecome a material that will be in greater demand than these rivalmetals.

Titanium metal has poor processability and cutting property, andtherefore using dispersed titanium material in a production ofmechanical parts having a complex shape will require additionalmachining steps such as cutting after hot forging, rolling or otherplastic working process. This inevitably increases process steps andadds to production costs.

For the above reason, powder metallurgy is often used in applications inwhich titanium metal is used, as mentioned earlier, and accordinglythere is demand for titanium powder that offers high purity and uniformparticle shape and size. When titanium powder is produced using theconventional powder production methods applicable to general metals, theproduced titanium powder will pose problems, just like other metalsproduced in the same manners, in terms of irregularities in particleshape and size, poor economy, and so on. Therefore, development of amethod for producing titanium powder that can yield higher purity andmore uniform particle shape and size is eagerly awaited.

For example, improved production methods of titanium metal powder usingthe hydrogenation and dehydrogenation method and rotary electrode methodhave already found commercial use. The hydrogenation and dehydrogenationmethod is a technique to heat titanium sponge, dispersed titaniummaterial or titanium scraps generated from cutting/machining processesin a hydrogen atmosphere to cause the titanium material to absorbhydrogen gas, and then crush the embrittled titanium material, afterwhich the crushed titanium is again heated in a vacuum atmosphere torelease hydrogen gas and consequently obtain titanium powder. The rotaryelectrode method uses a round bar formed from dispersed titaniummaterial or processed dispersed titanium material, which is a forged,rolled or otherwise processed version of dispersed titanium, material.This round bar material is rotated at high speeds in an atmosphere ofinactive gas such as argon or helium while its tip is dispersed using aheat source such as an arc or plasma arc, and the dripping molten metalis spattered by centrifugal force to obtain spherical powder particles.In the rotary electrode method, controlling the dispersion amount of thedispersing metal is very difficult.

Titanium powder obtained by the hydrogenation and dehydrogenation methodhas irregular sphericity. Therefore, although such powder can be usedfor die molding, the heating process must be repeated twice. While amechanical crushing process using a ball mill may be devised, it willinevitably cause oxygen contamination of the titanium powder. On theother hand the rotary electrode method, wherein molten titanium materialis shaped into powder form in an inactive gas atmosphere, producesspherical powder particles with good fluidity and there is no risk ofoxygen contamination. However, this method has a drawback of poorsolidification of molding material. In addition, both methods use batchprocessing and therefore the powder production costs are higher.

The atomization method was developed as a titanium powder productionmethod aiming to resolve the aforementioned problems relating to qualityand production costs. Under the atomization method, metal material isdispersed in a water-cooled copper crucible using a plasma arc or otherheat source and the molten metal is caused to drip from one end of thecrucible. Then, an inactive gas such as argon or helium is injected tothis molten metal drips to atomize the molten metal and obtain powder.However, this method couldn't achieve significantly lower productioncosts compared with the conventional methods, since it also useddispersed titanium material and processed dispersed titanium material.

The invention described in Japanese Patent Application Laid-open No.5-93213 presents a method for producing titanium powder that offersimproved sphericity and fluidity to facilitate molding, in a mannerrequiring less production costs and avoiding oxygen contamination.However, this method, wherein titanium sponge is isostatically pressedin cold state and the solidified bar material is melted in an inactivegas atmosphere, and then an inactive gas such as argon or helium isinjected to atomize the molten metal to obtain powder, still didn'tprovide powder of desired levels of purity as well as uniform sphericityand particle size and the production costs were not ideal, either.

DISCLOSURE OF THE INVENTION

Despite the growing needs and demands for metal powder—especiallytitanium metal powder—as mentioned above, on the back of advancement innew molding/processing methods such as powder metallurgy, the powderproduction methods developed to date have not been able to fully addressthe requirements for production of such metal powder. In particular,these methods posed problems in the purity of elemental metal,uniformity of sphericity and particle size, and production costs.

In view of the above situation, the present invention aims to produceand provide powder material offering excellent uniformity of sphericityand particle size and high purity of elemental metal, for use in moldingprocesses such as powder metallurgy.

After studying numerous ways to improve the problems associated with theproduction of elemental metal powder such as titanium powder, includingpoor purity of elemental metal, irregularity of sphericity and particlesize, and high production costs, the inventors have successfully solvedthe aforementioned problems by utilizing the technology filed earlier bythe inventors relating to a production of high-function water containingtitanium (Japanese Patent Application No. 2001-315446).

The aforementioned earlier invention concerning a production ofhigh-function water containing titanium (Japanese Patent Application No.2001-315446) relates to a method for producing high-function water inwhich titanium metal is micro-dispersed by means of causing plasmadischarge in water between a titanium metal electrode and its counterelectrode and then causing the generated metal ion vapor to contact, anddisperse in, water. The present invention utilizes this technology toallow for production of elemental metal powder, especially titaniummetal powder, offering excellent purity and uniformity of sphericity andparticle size, at a significantly lower production cost.

The method and apparatus proposed by the present invention arefundamentally different in concept and structure from the conventionalproduction methods for metal powder and titanium powder. Basically, thepresent invention aims to obtain metal powder as settlements in water bycausing plasma discharge in water and thus converting elemental titaniummetal into fine particles. This technique can also be applied to metalsother than titanium, and the production method and apparatus proposed bythe present invention indeed embody a notable improvement in metalpowder production based on a completely different approach from thoseadopted by the conventional methods.

Specifically, the present invention, wherein the basic concept is tocause plasma discharge in water between an elemental metal electrode andits counter electrode and then cause the generated metal ion vapor tocontact water and become powder form, comprises (1) through (7)specified below:

(1) A method for producing metal powder, wherein plasma discharge iscaused in water between an electrode made of elemental metal and itscounter electrode and the generated metal ion vapor is caused to contactwater and become powder form.

(2) A method for producing metal powder as described in (1) above,wherein the elemental metal is titanium, zirconium, germanium, tin,gold, platinum or silver.

(3) An apparatus for producing metal powder, which comprises a powersupply for high-voltage/current discharge, an elemental metal electrodefeeder, a high-voltage discharge generator equipped with an elementalmetal electrode and its counter electrode, a water tank, a water inletto the water tank, an outlet for produced dispersion water of fineelemental metal particles, a discharge pump, and a filter system.

(4) The apparatus for producing metal powder as described in (3) above,wherein titanium, zirconium, germanium, tin, gold, platinum or silver isused as the elemental metal.

(5) The apparatus for producing metal powder as described in (3) or (4)above, wherein the elemental metal constituting the electrode has a bar,plate or wire shape.

(6) The apparatus for producing metal powder as described in any one of(3) through (5) above, wherein one electrode is made of elemental metaland its counter electrode is made of carbon and a pair of the electrodesare vibrated or slid to prevent fusion between the electrodes, andwherein instant plasma discharge is generated to control the amount ofdispersion.

(7) The apparatus for producing metal powder as described in any one of(3) through (6) above, wherein the amount of current flowing through thecircuit can be easily adjusted by changing the diameter and/or length ofthe carbon electrode.

The method and apparatus proposed by the present invention allow forproduction of elemental metal powder in a very efficient manner. Inaddition, the present invention does not generate any byproducts orimpurities other than the target metal powder. The generation of metaloxide due to heating of the metal material is also very small, theobtained metal powder particles have excellent uniformity in theirsphericity and size, and the production costs can be loweredsignificantly. Continuous production is also possible, in addition tobatch production, so metal powder of uniform particle size can beproduced in mass volumes at an economy that sufficiently meets therequirement for such commercial production.

In the production process proposed by the present invention, plasmadischarge is caused in water between an elemental metal electrode andits counter electrode to obtain ion vapor of the elemental metal. As thevapor contacts water, it instantly disperses in water as fine particlesto become fine powder. In addition, by comprising the counter electrodeused in such underwater plasma discharge not from the same metal as theelemental metal electrode but from carbon, and also by vibrating orsliding the electrode pair, fusion between the electrodes can beprevented. Furthermore, achievement of instant plasma discharge makes iteasy to control the amount of dispersion, and there is no need to selecta different power supply for a given purpose because the amount ofcurrent flowing through the circuit can be easily adjusted by changingthe diameter and length of the counter electrode made of carbon. Carbonparticles that disperse simultaneously with metal particles are harmlessand nearly the entire amount can be removed easily using a filtersystem, thereby enabling a production of metal dispersion water of highpurity. Through these processes the fine particles of the elementalmetal used as an electrode are obtained as proposed by the presentinvention.

According to the present invention, fine metal powder of zirconium,germanium, tin, gold, platinum or silver can be produced in addition totitanium powder, by using a desirable metal as the elemental metalmaterial.

The basic structure of the present invention provides a method forproducing metal powder of uniform particle size by causing plasmadischarge in water between an electrode made of elemental metal and itscounter electrode made of carbon, etc., and then causing the generatedmetal ion vapor to contact water and become powder form, as explainedabove. The production flow chart shown in FIG. 1 outlines thisproduction process.

As shown in FIG. 1, distilled water or other demineralized water isfilled in a water tank used for titanium metal powder production. Then,an electrode made of titanium metal bar, etc., is fed by a feeder ofelemental metal electrode, and plasma discharge is caused in waterbetween the elemental metal electrode and its counter electrode made ofcarbon bar. When the elemental metal ion vapor generated by underwaterdischarge contacts water, the vapor instantly disperses in water. Atthis time, very small fine titanium particles of micron-scale areproduced and dispersed as powder to form dispersion water of theelemental metal. This fine elemental metal powder in water does not meltor float and instead settles after a brief period. This powder can berefined by filtering to obtain fine elemental metal powder. The obtainedfine elemental metal powder has high purity as well as uniformsphericity and particle size.

BRIEF EXPLANATION OF THE DRAWINGS

“FIG. 1”

A production flow chart of metal powder as proposed by the presentinvention

“FIG. 2”

An apparatus for producing metal powder as proposed by the presentinvention

EXPLANATION OF THE SYMBOLS

-   1. Plasma discharge generator-   2. Power supply for high-voltage/current discharge-   3. Electrode-vibrating/sliding device-   4. Elemental metal electrode feeder-   6. Elemental metal electrode-   7. Counter electrode-   8. Water inlet-   9. Outlet for elemental metal dispersion solution-   10. Discharge pump-   11. Filter system-   12. Filtrate-   13. Metal powder-   14. Water tank

BEST MODE FOR CARRYING OUT THE INVENTION

An example of titanium metal powder production is explained below. Note,however, that the present invention is not limited to a production oftitanium powder.

Although the present invention allows for production of pure titaniumpowder in a very efficient manner, controlling the feed rate of theelectrode made of titanium metal is very important in achieving suchefficient production of pure titanium powder. For example, the amount ofcurrent flowing through the circuit can be adjusted by changing thediameter and length of the carbon counter electrode as one such means.

According to the production apparatus proposed by the present invention,plasma discharge is caused in water inside a water tank. Therefore, awater tank with sufficient pressure resistance that can withstand thehigh pressure required in underwater plasma discharge is needed.

In addition, by comprising the counter electrode used in discharge notfrom the same metal as the elemental metal electrode but from carbon,and also by vibrating or sliding the electrode pair, fusion between theelectrodes can be prevented. Furthermore, achievement of instant plasmadischarge makes it easy to control the amount of dispersion, and thereis no need to select a different power supply for a given purposebecause the amount of current flowing through the circuit can be easilyadjusted by changing the diameter and length of the counter electrodemade of carbon. Carbon particles that disperse simultaneously withelement metal particles are harmless and nearly the entire amount can beremoved easily via filtering, thereby enabling a production of elementalmetal dispersion water of high purity.

The electrode made of titanium metal material may have a bar, plate orwire shape. In the case of a small production tank with a capacity muchsmaller than one ton, it will be more appropriate to introduce titaniummetal as a wire, instead of a bar.

Other than titanium, the elemental metal materials from which metalpowder can be produced using the production apparatus proposed by thepresent invention include zirconium, germanium, tin, gold, platinum andsilver. However, possible applications of the present invention are notlimited to these elemental metals.

An example of the present invention is explained according to thedrawings. Note, however, that the present invention is not limited tothis example.

FIG. 1 shows a production flow chart of metal powder according to thepresent invention as explained above.

FIG. 2 illustrates an apparatus for producing metal powder proposed bythe present invention, which comprises a water tank (14), a plasmadischarge generator (1) equipped with an elemental metal electrode andits counter electrode, and a filter system (11) for filtering elementalmetal powder.

A pressure-resistant container used for metal powder production isequipped with a power supply for high-voltage/current discharge (2), adevice for vibrating or sliding electrodes (3), a device for feedingelemental metal electrode (4), the plasma discharge generator (1)equipped with an elemental metal electrode (6) and its counter electrode(7), a water inlet (8) to the water tank (14), an outlet (9) forelemental metal dispersion solution generated by plasma discharge inwater, a discharge pump (10), and the filter system (11) for separatingmetal powder from the elemental metal dispersion solution. Producedmetal powder is denoted as (13).

Demineralized water is fed into the water tank installed in the plasmadischarge generator.

Plasma discharge is caused between the elemental metal electrode made oftitanium and its counter electrode made of carbon, which are bothsubmerged in water inside the tank. Underwater plasma dischargegenerates titanium ion vapor, and when this vapor contacts water adispersion solution of titanium metal is produced.

As for plasma discharge, fusion between the electrodes can be preventedby vibrating or sliding the electrodes using the sliding/vibratingdevice (3), and achievement of instant plasma discharge makes it easy tocontrol the amount of dispersion. In addition, the titanium metalelectrode is fed continuously or intermittently using the electrodefeeder (4) so as to ensure the electrode is consumed sequentially by thenecessary amount. Underwater plasma discharge instantly melts thetitanium material and causes the molten titanium to disperse in water.

In this process, very small fine titanium particles of micron-scale (4)are generated and dispersed as powder. The generated titanium metalpowder doesn't melt or float and instead settles as powder after a briefperiod to separate from water. The obtained water is retrieved from theoutlet for titanium metal powder (9) and filtered through the filtersystem (11) into filtrate (12) and titanium powder (13). When 25 kg oftitanium metal bar was consumed in the water tank filled with one ton ofwater, the resulting water contained a small amount of dissolvedtitanium. However, the rest of the titanium electrode did settle at thebottom of the container as titanium powder. The average particle size ofthis titanium powder was 10 to 30 μm.

In addition, the obtained titanium powder contained no byproducts orimpurities and the titanium powder particles had very uniform sphericityand particle size.

The present method and apparatus indeed produced titanium powder ofuniform particle size in a very economical manner.

INDUSTRIAL FIELD OF APPLICATION

The present invention allows for production in a very efficient andstable manner of metal powder, especially titanium powder, offering highpurity. The production method proposed by the present inventioneliminates generation of byproducts and impurities other than the targetmetal content, and the obtained powder has excellent uniformity in termsof sphericity and particle size. Additionally, the compact, efficientapparatus achieves significant reduction in production costs.Furthermore, the present invention can be applied to batch production,continuous production and mass production.

1. A method for producing metal powder comprising: causing plasmadischarge in a high pressure water between an electrode made ofelemental metal and its counter electrode to generate metal ion vapor;and contacting the generated metal ion vapor with water to convert themetal ion vapor to metal powder.
 2. The method for producing metalpowder as described in claim 1, wherein the elemental metal is titanium,zirconium, germanium, tin, gold, platinum or silver.
 3. A method forproducing metal powder comprising: causing plasma discharge in waterbetween an electrode made of elemental metal and its counter electrodeto generate metal ion vapor; and contacting the generated metal ionvapor with water to convert the metal ion vapor to metal powder.
 4. Themethod for producing metal powder as described in claim 3, wherein theelemental metal is titanium, zirconium, germanium, tin, gold, platinumor silver.
 5. The method for producing metal powder as described inclaim 3, comprising vibrating or sliding said electrode and its counterelectrode to prevent fusion between the electrodes, and generatinginstant plasma discharge to control the amount of dispersion of saidmetal ion vapor.
 6. The method for producing metal powder as describedin claim 3, comprising adjusting the amount of current flowing through acircuit by changing the diameter and/or length of said counterelectrode.